Excess vacancy generation mechanism at phosphorus diffusion into silicon

Abstract

Phosphorus is diffused into silicon at 900 °C from a phosphorus-doped silicon-dioxide layer. Since a profile of phosphorus concentration is expressed by a function of x/t, where x is the distance from a surface and t the diffusion time, diffusion coefficients are determined by the Boltzmann-Matano method. They are larger than the intrinsic diffusion coefficient and are dependent not only on the concentration (the concentration effect) but also on some unknown condition at a surface (the surface effect). The surface effect extends more than 20 μ deep into a bulk of silicon, and is stronger than the concentration effect. All of phosphorus atoms are located at substitutional sites. Diffusion-induced dislocations are not found. A new mechanism for the generation of excess vacancies is suggested. The new mechanism consists of the following: (i) Phosphorus diffuses by a vacancy mechanism. The diffusion of phosphorus occurs only through the diffusion of E centers. (ii) When phosphorus atoms enter from a surface into a bulk, they should be in a form of E centers. Affected by a surface, a large amount of E centers is formed per unit time at a surface. (iii) The E centers flow into a bulk. (iv) By their dissociations, excess vacancies are generated. The surface effect and the emitter dip effect are attributed to excess vacancies.